The present invention relates to an apparatus for welding plastic parts, and more specifically, for devices to weld plastic parts using Through Transmissive Infrared Radiation (TTIR).
Typically, plastic assemblies can be welded together by one of the many methods for welding or joining plastics currently available. These methods include ultrasonic welding, adhesives, fasteners, friction, hot plates, induced welding, microwave welding, or radiant heaters. Although these methods sometimes produce welds of adequate strength, often they are not adequate and each has some drawbacks. For example, vibration during frictional welding can damage installed electronic devices or the plastic pieces. Furthermore, part size and positioning accuracy are limited because one part must move relative to the other during the procedure. Melt residue deposited on hot-plate tooling requires frequent cleaning. On the other hand, the relative high temperatures required for non-contact, radiant heaters could degrade materials at the weld joints or overheat sensitive electronics.
Other methods used to weld plastics include the use of coherent light sources like lasers (Nakamata), or non-coherent light sources like halogen lamps (Grimm et al).
Nakamata (U.S. Pat. No. 4,636,609 “Process for joining different kinds of synthetic resins.”) shows a TTIR method of joining different plastics by using a substrate plastic doped with an absorbing element (e.g. carbon) to decrease the transmissivity of the plastic and thus absorb the energy, produced by the radiant source. The energy absorbed by the substrate plastic generates enough heat to bring the component to its melting point. Because the substrate plastic is in intimate contact with another piece of the same material (but without an absorbing element), when the substrate melts, so does the other component (this component is typically transparent). Radiant energy from the monochromatic source (a laser) is directed through the transparent plastic to the substrate plastic. Heating in the substrate plastic is transferred to the transparent plastic by conduction in the area where both components meet. When both the substrate and the transparent component are melted, the two pieces are pressed together, the source of radiation is terminated, and then the assembly is allowed to cool producing a satisfactory weld.
A laser is the only radiant source specified in this patent. A neodymium-doped YAG (Yttrium Aluminum Garnet) laser with a radiant output at a wavelength of 1060 nm (nanometers) is the most suitable according to the inventor. A neodymium-doped glass laser, a ruby laser, a helium-neon gas laser, a krypton gas laser, an argon gas laser, a hydrogen gas laser, or a nitrogen gas laser may also be used. Nakamata contends that laser radiation sources with a wavelength of 1060 nm (the neodymium-doped YAG wavelength) or less is necessary. Wavelengths longer than the aforementioned 1060 nm cannot be transmitted through the otherwise transparent plastic according to the inventor.
Nakamata teaches that 5 to 100 watts of laser power are necessary to affect satisfactory welds. No melting occurs at power levels less than 5 watts, and laser power levels in excess of 100 watts may vaporize or significantly alter the properties of the transparent plastic.
Grimm is the inventor of two related TTIR method patents (U.S. Pat. Nos. 5,840,147 and 5,843,265) assigned to the Edison Welding Institute and licensed to Quantum Group Inc. Grimm utilizes a similar scheme as Nakamata to bond two plastic pieces. That is, a base layer of plastic is absorbing and is heated by incident radiation. A second plastic piece, which is to be bonded to the base layer, is essentially transparent to the radiation employed and is placed on top of the base layer. Incoming radiation, as in Nakamata, is routed through the almost transparent top piece and into the absorbing base layer. Bonding occurs when the base piece melts at the interface with the top piece. Pressure is applied to press the two parts together, the radiation is terminated, and the assembly is allowed to cool. Grimm's bonding method is similar to Nakamata's except for the radiation source. Nakamata employs a laser, and Grimm uses a source of non-coherent electromagnetic radiation.
Grimm utilized a source of, non-coherent electromagnetic radiation, a quartz-halogen-tungsten lamp, which can be described as an approximately gray body emitter. Grimm cites a problem with his radiation source, his source is a broadband emitter that generates substantial amounts of long wavelength IR (Infrared) that are absorbed in his supposedly transparent plastic piece. This causes the same problem that Nakamata . experienced in his transparent plastic with high power lasers (power levels greater than 100 watts). Grimm employs the inventive step of utilizing an extra piece of transparent plastic placed between his source and the work piece to absorb the long wavelength energy. Grimm devises a method of cooling (forced convection) this extra piece of plastic to dissipate the heat generated in this kind of filtering technique.
The technique of Nakamata and Grimm may be employed for critical bonding applications providing the thermal energy delivered by the welding process can be limited. A very narrow bond line is a practical method of achieving hermeticity at limited power levels. Nakamata's sealing method can provide a narrow bond line because of the non-diverging nature of a laser beam (i.e. a laser with a wavelength of 808±10 nm has a 0.8 mm beam diameter). Grimm, however, provides a wider beam by using a lamp and reflector system with the weld at the focal length of the lamp/reflector system. In both of these approaches, either the optical source or the work piece (or both) must move in the appropriate trajectory to accomplish the bond at the required location. The correct amount of compression force for the requisite amount of time must follow the beam, and this requires complicated, automated tooling or robotic systems. In this approach, furthermore, there is an inherent discontinuity at the starting point and at the ending point of the weld, which may effect the quality of the product. Goldstein and Tolley disclosed a TTIR method using a photon reflecting mask in U.S. patent application Ser. No. 60/116,575 Filed Jan. 21, 1999 and 09/488,887 Filed Jan. 21, 2000.